add regression test for #30225

Fixes #30225

Author: nikomatsakis

src/test/compile-fail/issue-30225.rs

@@ -0,0 +1,48 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test for #30225, which was an ICE that would trigger as
+// a result of a poor interaction between trait result caching and
+// type inference. Specifically, at that time, unification could cause
+// unrelated type variables to become instantiated, if subtyping
+// relationships existed. These relationships are now propagated
+// through obligations and hence everything works out fine.
+
+trait Foo<U,V> : Sized {
+    fn foo(self, u: Option<U>, v: Option<V>) {}
+}
+
+struct A;
+struct B;
+
+impl Foo<A, B> for () {}      // impl A
+impl Foo<u32, u32> for u32 {} // impl B, creating ambiguity
+
+fn toxic() {
+    // cache the resolution <() as Foo<$0,$1>> = impl A
+    let u = None;
+    let v = None;
+    Foo::foo((), u, v);
+}
+
+fn bomb() {
+    let mut u = None; // type is Option<$0>
+    let mut v = None; // type is Option<$1>
+    let mut x = None; // type is Option<$2>
+
+    Foo::foo(x.unwrap(),u,v); // register <$2 as Foo<$0, $1>>
+    u = v; // mark $0 and $1 in a subtype relationship
+    //~^ ERROR mismatched types
+    x = Some(()); // set $2 = (), allowing impl selection
+                  // to proceed for <() as Foo<$0, $1>> = impl A.
+                  // kaboom, this *used* to trigge an ICE
+}
+
+fn main() {}